1. Field of the Invention
This invention relates generally to wireless systems. More particularly, it relates to a wireless telephone system (e.g., a cellular telephone) which transmits location information to a called party.
2. Background of Related Art
Wireless telephone systems are becoming available everywhere, and are now in wide use by many people. A wireless telephone system such as a cellular telephone system allows users to make and receive telephone calls via a portable handset.
Many cellular telephone systems provide a public service by allowing a user to dial special telephone numbers to, e.g., report traffic accidents, call police, etc. Perhaps most importantly cellular telephone systems allow a user to dial an emergency telephone number such as ‘911’.
In a wireline system, a 911 telephone call includes call related information to the called party (e.g., a street address of the caller) allowing emergency personnel to rapidly arrive at the correct address. Address information is transmitted in a 911 capable system to the called emergency center regardless of the ability of the caller to speak, giving emergency personnel crucial identity and address information even in the most extreme emergency cases where the caller is unable to speak.
The functions of 911 systems have become well known in the wireline telephone system, but use of a 911 call using a wireless telephone such as a cellular telephone presents its own challenges. For instance, since cellular telephones are able to roam just about anywhere, address information is not indicative of the location of the cellular telephone at any one time. Thus, at best, when a 911 call is placed in conventional cellular telephone systems the location of the fixed component of the cellular telephone system, i.e., the location of the base station servicing the cellular telephone at the time of the 911 telephone call, is all the location information that can be transmitted to the emergency personnel. However, since base stations usually service cellular telephones within a very large area (e.g., many square miles), a 911 system is rather impractical using conventional cellular telephones.
A conventional solution which provides more useful location information to a called party includes a global positioning satellite (GPS) within a cellular telephone handset. FIG. 3 shows such a conventional cellular telephone 300 including cellular telephone functionality 302 combined with a GPS system 304. Using the GPS system in the cellular handset, location information can be determined using the GPS, then transmitted to a called party during a 911 telephone call.
A 911 system in a cellular telephone system carries great potential. For instance, a caller who is moving may stay on the line while emergency personnel are en route to reach them, and the cellular telephone may provide updated location information to the emergency personnel as the 911 caller moves.
Ordinarily, a GPS system is at best accurate to about 10-15 meters (e.g., 30 to 45 feet). Certain factors such as what is known as the ionosphere delay effect can affect the accuracy of the GPS system in any particular location at any particular time. In a wide-open area, a 10-15 meter accuracy is close enough for emergency personnel to locate the calling party. However, in many situations (e.g., in a crowded building, on a crowded street, etc.), a 15 meter may not direct emergency personnel.
To make matters worse, a GPS system may be adjusted to have less accuracy, e.g., about 100 meters (e.g., 300 feet). For instance, in the United States, the Department of Defense may adjust the GPS system using selective availability (SA) to have a less precise accuracy at any time (e.g., when the national interest outweighs commercial use of the GPS system).
Thus, the accuracy of the location of the user transmitted during a 911 call is limited to the accuracy of the GPS system at any one time. Moreover, as the accuracy of the GPS system is manipulated, so is the ability of emergency personnel to quickly locate persons making 911 telephone calls from GPS equipped cellular telephones. During such times, use of a GPS system to provide location information to emergency personnel during a 911 telephone call becomes somewhat impractical or useless.
There is a need for a wireless telephone system such as a cellular telephone system which is capable of transmitting more accurate location information to a called party or other emergency personnel not only when the GPS and/or Global Navigation Satellite System (GLONASS) systems are operating in a less accurate mode, but also when the GPS and/or GLONASS systems are operating in a typical accuracy mode (e.g., 10-15 meters).